Ive only ever smoked caapi bowls prior to the changa, but here is my input. You arent that far from smoking changa the way you are doing it.
I find changa gives me alot of "body" feeling, like energy rushing through me, somthing i dont get with Spice.
Open-eyes at a certain level can be quite confusing, I dont like it much, but closed-eyes at the same level is a different ball game, much better IMO. Breakthrough to me is the same OPE or CEV, there is no difference, all reality melts away, here is "there". At least thats what i think, cant be sure of what my eyes are doing during a breakthrough.
Im sure at a higher dose you would have busted through, but with changa I feel that is part of the journey, you gotta work through it.
One can just carry on smoking the leaf and spice all night, if thats what you wish to do, I dont think that the leaf would "build up" but it would work. Ive never experimented with how long the harmalas stays in the body after each bowl, but I do know that you can just keep pulling away as long as you wish.
Safe travels and report back